Dissolution of metal with acidified hydrogen peroxide solutions



United States Patent 3,407,141 DISSOLUTION OF METAL WITH ACIDIFIEDHYDROGEN PEROXIDE SOLUTIONS Russell Shukri Banush and Donald PaulHagerty, Onondaga, N.Y., assignors to Allied Chemical Corporation, 'NewYork, N.Y., a corporation of New York No Drawing. Filed Feb. 3, 1966,Ser. No. 524,836 21 Claims. (CI. 25279.4)

ABSTRACT OF THE DISCLOSURE Relates to acidified hydrogen peroxidesolutions and method for dissolving metals, particularly useful in themanufacture of printed circuit boards, wherein said acidified hydrogenperoxide solutions contain an additive selected from the groupconsisting of phenylurea, diphenylurea, benzoic acid, hydroxy benzoicacid and salts and mixtures thereof.

Dissolution or etching of metals for a constructive purpose as in themanufacture of printed circuits is not a simple matter and involvesseveral considerations if it is to be a practical success. Among themore important considerations are rate of attack of the etchant, controlof the etchant, stability and efficiency of the etchant, time andtemperature conditions, effect of the etchant on the materials formingthe printed circuit board, and effect on the equipment and maskingmaterial employed in the etching process. In order to be practical achemical etchant must have capacity to etch a relatively large amount ofmetal before the etch rate is slowed to an impractical level by theeffective exhaustion of the etchant. Each increment in the amount ofmetal etched represents a significant increase in efficiency and asubstantial reduction in the cost per weight unit of metal etched.Etching of metals such as copper has been carried out with an aqueousferric chloride solution which has been satisfactory at least in itsability to efficiently etch copper without material adverse sideeffects. However, in more recent times, the disposition of the spentferric chloride etchant solution containing both iron and copper hasbecome a problem, largely because of the disposal of the waste liquorand difficulty in recovering copper therefrom which is of coursedesirable from a cost standpoint. Other copper etchants have thereforebeen sought with the result that aqueous solutions of ammoniumpersulfate have been adopted by some users. This etchant permitselectrolytic recovery of copper from the spent etchant solution andeliminates the problem of disposing of metal containing waste liquors.However, ammonium persulfate as an etchant is a premium material becauseof its low etching capacity and is subject to other drawbacks which haveleft considerable room for improvement in the provision of an etchantfor copper.

As an etchant for copper, aqueous hydrogen peroxide is very attractivebecause of its relatively low cost and ability to recover copperelectrolytically from a spent peroxide etchant solution. However, theutilization of hydrogen peroxide for a constructive purpose in metaletching is subject to numerous problems and pitfalls. Basically,potentially useful solutions combining hydrogen peroxide and acid werefound subject to great deficiencies by way of etch rates and capacity.Recently, ourselves and our associates have participated in thedevelopment of acidified hydrogen peroxide solutions and conditions ofuse thereof whereby metal may be dissolved or etched for constructivepurposes in a practical manner and at considerable savings overconventional etching solutions. Such solutions may contain additivessuch as phenacetin, sulfathioazole and silver ions and satisfy allrequirements necessary to constitute new and improved 3,407,141 PatentedOct. 22, 1968 etchants for even copper metal, including unexpectedlyfast etch rates and high capacity. In practical use these etchants aredesigned to be prepared from concentrates and after preparation aresubject to a loss of maximum effectiveness with the passage of time,typically a depreciation of 10% over a period of 12-36 hours. Etchantsof improved capability of storage without substantial depreciation aredesirable for convenience of the user in permitting greater flexibilityin preparation of the solutions and avoiding loss of effectivenessduring extended periods of shutdown of operations.

An object of the present invention is to provide a new and improvedetchant based on hydrogen P roxide.

Another object is to provide new acidified peroxide etchants especiallyhaving improved capacity for standing for extended periods of time afterpreparation without substantial loss of effectiveness.

Another object is to provide new and improved hydrogen peroxide etchantscapable of dissolving large amounts of metal at fast rates.

A further object is to provide new and improved acidperoxide solutionsfor etching copper for a constructive purpose as in the manufacture ofprinted circuits.

A still further object is to provide hydrogen peroxide etchants adaptedfor eflicient practical use in various conventional etching apparatusand procedures including both immersion and spray etching operations.

Other objects and advantages will be evident from the followingdescription of the invention.

In accordance with the present invention it has been found thatacid-hydrogen peroxide solutions having high capacity and capable ofetching metals at fast rates are provided by incorporating in suchsolutions a small amount of one or more additives selected from thegroup consisting of phenylurea, diphenylurea, benzoic acid and hydroxybenzoic acid. Solutions containing such additives not only etch metalsincluding copper at high rates and with high capacity but also exhibitexceptionally good storage life without substantial depreciation afterpreparation over periods of 4-10 days, or even greater. It has been alsofound that acidified hydrogen peroxide solutions having further andmarkedly improved etch rates and capacity are provided by incorporatingtherein a combination of one or more Of the above-indicated additiveswith a small amount of at least one additive selected from the groupconsisting of sulfathioazole, phenacetin and silver ions. Theparticularly preferred solutions provided by the present inventioncontain a combination of phenylurea and sulfathiazole and arecharacterized by ability to etch large amount of metal at very fastrates and also ability to be stored or permitted to stand over extendedtime periods after preparation without substantial loss of maximumeffectiveness. All the acidified peroxide solutions provided by theinvention are effective in dissolving copper metal and may be employedin accurate, con trolled and highly efficient etching of copper-cladlaminates as in the manufacture of printed circuit boards.

Phenylurea, diphenylurea, benzoic acid and hydroxy benzoic acid are allhighly effective in improving the etch rate and capacity ofacid-peroxide solutions. Salts yielding these additives in theacid-peroxide solution may also be employed. For example, the sodiumsalt of benzoic acid and hydroxy benzoic acid may be added to thesolution. The preferred additive is phenylurea. In preparation of theacidified peroxide solutions of the invention, only a small amount ofthe above additives is required to have the desired catalytic affect onetch rate and capacity. The phenylurea and benzoic acid compounds may beadded in an amount as little as 30 parts per million to provide anetchant of improved capacity etch rate. Increasing the amount ofadditive will further increase etch rate and capacity. About 100-1000parts per million of the phenylurea and benzoic acid additives ispreferred. The upper limit of the amount of additive is not critical andmostly a matter of economics. Generally, an amount of additive in excessof about 5,000 parts per million offers no added advantage and is lessdesirable from a process and economic standpoint.

Acid-hydrogen peroxide solutions of exceptionally fast etch rates arealso provided by the invention by incorporating in the solutions acombination of additives involving one or more of phenylurea and benzoicacid compounds with one or more additives from the group ofsulfathiazole, phenacetin and silver ions. In the solutions containingsuch combinations as little as 20 parts per million of each additive maybe employed. Somewhat lesser amounts of free silver ion may be employed,particularly in immersion etching procedures where as little as aboutparts per million would be effective. The particularly preferredadditive combination is phenylurea and sulfathiazole with the phenylureapreferred over diphenylurea in such combination by reason of differencebetween cost of the compounds. Silver ions may also be incorporated inthe phenylurea-sulfathiazole combination to further improve the etchrate performance of the solution. Combination of benzoic acid compoundsand silver ions are also effective in producing etchants having fastetch rates and high capacity substantially improved over benzoic acidalone. The preferred total amount of additives in the combinationsystems is between about 150- 1500 parts per million with between about50-500 parts per million of each being employed. Hence, the particularlypreferred solutions have incorporated therein between about 50-500 partsper million phenylurea and between 50-500 parts per millionsulfathiazole. Another preferred solution contains benzoic acid andsulfathiazole each in an amount of 50-500 parts per million.

In preparation of acid-hydrogen peroxide solutions for treatment ofcopper metal special consideration must be given to provide solutionswhich contain less than 2 parts per million total free chloride andbromide ions, preferably less than 1 part per million. For example,deionized water may be used to make up an etchant containing less than 2parts per million of chloride and bromide ions. Or, if desired, ordinarywater may be employed in make up of the etchant solution if accompaniedby additions of suitable material capable of removing free chloride andbromide ions. For example, a small amount of water-soluble silver salt,preferably silver nitrate, may be added to effect the removal ofchloride and bromide ion. The precipitated silver halide matter isallowed to remain in the acid-peroxide solution and does not interferewith the etching process. The addition of excess soluble silver saltwill furnish free silver ions in the etchant and have a highlybeneficial and catalytic effect upon each rate and capacity. Whensulfathiazole is employed in the etchant it has been found that specialconsideration may be dispensed with. While the explanation for thisresult is uncertain, it is evident that sulfathiazole functions not onlyto increase the etching capacity of peroxide solutions but also tonegate the adverse repressive efiect of the chloride and bromide ionconcentration on etch rate and capacity. Hence, the addition ofsulfathiazole may permit use of ordinary tap water in preparation of theetchants. Salts yielding sulfathiazole may also be employed and aregenerally preferred because such compounds are more easily dissolved inthe acid-peroxide solutions. The sodium salt of sulfathiazole is anexample of preferred salt which may be added to the solutions. Whenemploying sulfathiazole for this purpose in etchants prepared fromordinary tap water between about 150 to 250 parts per million is usuallyrequired to negate the adverse effects of the free chloride and bromideions in the water. The capacity of sulfathiazole in overcoming theadverse effect of chloride and bromide ion is apparently not unlimited.Solutions containing the higher concentrations of these ions,

4 say above about 20-30 parts per million, will require additionaltreatment to negate this effect of such ions, e.g. deionization oraddition of a soluble silver salt.

In the dissolution of metal by the invention the hydrogen peroxideconcentration may vary over a fairly wide range. Etching of metal isdesirably carried out in acidified solutions having a hydrogen peroxideconcentration between about 2-l2%. At solution concentrations less thanabout 2% by weight etch rates are impractically low and etchingunsatisfactory. At concentrations about above 12% by weight, it has beenfound that copper metal may be etched but the dissolution of the etchedcopper ions in the etchant causes decomposition of the peroxide with theresult that etching at such high concentrations is less economical. Thebest results are obtained in solutions having a peroxide concentrationbetween about 210%. During the etching process, hydrogen peroxide isconsumed as more and more amounts of metal are treated. In order to bepractical it is necessary that a single etchant dissolve a substantialamount of metal before the solution becomes exhausted to the extent thata particular workpiece cannot be etched within a reasonable time, e.g.1-2 hours. The hydrogen peroxide solutions employed in the inventionmust therefore have an initial hydrogen peroxide concentration of atleast about 4% in order to dissolve sufficient metal to be practicalfrom an economic standpoint. Desirably, the etchant solution hasinitially a hydrogen peroxide concentration within the range of about5-10% by weight. The hydrogen peroxide solutions having the indicatedinitial hydrogen peroxide concentrations are useful in etching a singlelarge metal piece or a series of workpieces containing limited amountsof metal. The etchant is capable of operating effectively at good etchrates after partial exhaustion and at high dissolved concentrations ofmetals including copper in amounts equivalent to at least 8 ounces ofcopper per gallon and even substantially higher.

The acid concentration may also vary considerably. In etching it isdesirable that the etchant solution have a hydrogen ion concentrationfrom about 0.45 to about 5.5 grams per liter, preferably between about0.65-4.5 grams per liter. Below a hydrogen ion concentration of about0.45 grams per liter the etch rate is slow and peroxide decompositionhigh, particularly after partial exhaustion of the peroxide bath. Thedesired upper limit of the hydrogen ion concentration may depend onseveral factors including the particular acid employed. A hydrogen ionconcentration above about 5.5 grams per liter is generally lesseconomical and tends to slow down rather than increase the etch rate.Inorganic acids and even the stronger organic acids such as acetic acidmay be used to supply the hydrogen ion concentration in the etchantsolution. Examples of the acids whch are the more suitable includesulfuric acid, nitric acid, and fluoboric acid, preferably sulfuricacid. The amount of sulfuric acid in the hydrogen peroxide etchant isbetween about 2-23% by weight, preferably between about 3-20% by weight.Sulfuric acid concentrations above about 23% are less desirable incopper etching as tendency to result in less uniform dissolution. Thiseffect is apparently caused by the formation of a protective coating onsubstantial portions of the exposed copper surface which is thereby maderesistant to etching. The influence of the acid concentration on thecopper etch rate has been found of interest. When the acidified hydrogenperoxide etchant solution contains only minor amounts of dissolvedcopper and the effect of acid concentration on etch rate is negligibleand the full range of hydrogen ion concentrations between about 0.45 to5.5 grams per liter results in little variance in etch rate. As theperoxide bath becomes more exhausted and dissolved copper concentrationincreases, the infiuence of the acid concentration increases markedly.At the higher dissolved copper concentrations both the lower and higheracid concentrations results in longer etch times. An optimum etch ratehas been found to be reached at an intermediate hydrogen ionconcentration between about 0.9 to 1.4 grams per liter (about 4-6% byweight sulfuric acid). In the etching of metals such as copper thehydrogen peroxide and acid are theoretically consumed at a rateequivalent to a mol ratio of hydrogen peroxide. Thus, according to theetching reaction one mol of sulfuric acid is consumed for each mol ofperoxide and the acid concentration slowly decreases as the dissolvedcopper concentration increases. As the acid concentration does not havea substantial effect on etch rate at low dissolved copperconcentrations, it will be noted that the hydrogen peroxide etchant maycontain initially a high hydrogen ion concentration with relativelylittle sacrifice of etch rate after partial exhaustion and increase ofthe dissolved copper concentration. In situations where it is desired tooptimize etch rates and employ lower acid concentrations the etchantsolution may be advantageously made up to contain initially a low orintermediate hydrogen ion concentration, of the order of about OAS-3.4grams per liter (about 2-15% by weight sulfuric acid), preferablybetween about 1.1-2.6 grams per liter (about 512% by weight sulfuricacid). Then, as the etchant is consumed causing reduction of thehydrogen ion concentration additional acid is added to regulate thehydrogen ion concentration within the optimum range of about 0.9-1.4grams per liter (about 4-6% by weight sulfuric acid). Addition of theacid may take place either continuously or intermittently and eitherimmediately after the start of the etching or after significantexhaustion of the etchant solution. When the initial hydrogen ionconcentration is low, say of the order of about 0.45-l.l grams per liter(and 2-5% by weight sulfuric acid), the addition of the acid preferablytakes place substantially immediately after etching commences and isdesirably more or less continuous until the hydrogen ion concentrationis increased to well within the range of about 0.9-1.4 grams per liter.When the initial hydrogen ion concentration is greater than about 1.1grams per liter the addition of acid to maintain the optimumconcentration preferably takes place from time to time and after theetchant solution has been exhausted to the extent that the hydrogen ionconcentration is below about 1.1 grams per liter, usually just after theconcentration is reduced below about 0.9 gram per liter.

In the etchant solution the ratio of hydrogen peroxide to acid is lessimportant than the concentration of the acid. As the chemical reactionor mechanism by which copper is etched consumes one mol of peroxide and2 mols of acid hydrogen a mol ratio of 1 to 2 is indicated, i.e. a H O/H+ ratio of 1 to 2. Peroxide to hydrogen ion mol ratios less than 1 to2 are therefore generally unnecessary and may tend to slow the etchrate, particularly at the higher reagent concentrations. In practice,the amount of hydrogen peroxide actually consumed seldom will exceedabout 75% so that the inclusion of just slightly more than about 1.5mols of hydrogen ion per mol of peroxide will be adequate to supplysutficient acid for complete utilization of the particular etchantsolution. As some peroxide is also not utilized because ofdecomposition, the etchants made up to include sufficient acid forcomplete utilization without addition of more acid preferably have ahydrogen peroxide to hydrogen ion mol ratio of not less than about 1.0to 1.6, and desirably in the range of about 1.0:1.6 to 1.0 to 1.0. Whenacid is to be later added and the etchant solution contains initially alow or intermediate acid concentration, the mol ratio of peroxide toacid hydrogen may of course be initially somewhat greater, preferablybetween about 1.0202 to :10 As hydrogen peroxide is consumed and moreacid added, the mol ratio of peroxide to acid will be reduced andeventually become similar to the mol ratios preferably employed in thesolutions made up to contain the complete acid requirement. Again,because peroxide utilization seldom exceeds 75%, it is desirable from apractical viewpoint not to add an amount of acid sufiicient to reducethe mol ratio of peroxide to acid hydrogen below about 1.0 to 1.6.

Temperature of the acidified-hydrogen peroxide solution is anotherimportant factor in etching copper. As a practical matter, copper metalis not etched at room temperatures or below. The nature of the attack ofthe acid hydrogen peroxide solution on copper at such temperatures ismore of a polishing, oxidizing or brightening effect. In order toefficiently etch copper the hydrogen peroxide solution must have atemperature of at least about 40 C. at time of contact with the metal.Solution temperature has a strong effect on etch rates and increasingthe temperature to a preferred range between about 62 C. willsubstantially increase the rate of etching to a level significantlygreater than heretofore realized with ammonium persulfate etchants atrecommended optimum temperatures. At hydrogen peroxide solutiontemperatures above about 65 C. little further increase in etch rate isrealized and such temperatures have been found particularly undesirableas resulting in an impractically high rate of peroxide decomposition. Asis the case with acid concentration the influence of temperature on etchrate has been found to be greatest after partial exhaustion of theetchant and increase of the dissolved copper concentration. If desired,etching may be commenced at the lower temperatures, for example, betweenabout 40 C. to C., and temperature of the solution then graduallyincreased up to a higher temperature of approximately 55-62 C. as thesolution is further exhausted. Increasing the temperature of the etchantsolution is aided by the etching reaction itself which is moderatelyexothermic. increasing the temperature of the etchant may be used toadvantage to regulate etch rates at a more or less constant value when anumber of pieces are to be etched in the same solution such as, forexample, when employing auto matic systems used in the manufacture ofprinted circuits.

Generally, the etchant compositions of the invention may be prepared bysimple mixing of the required components. The etchants are mostconveniently and readily prepared from an aqueous hydrogen peroxideconcentrate containing between about 20-70%, preferably between 30-60%,by weight hydrogen peroxide and between about ZOO-20,000 parts permillion, preferably between 500- 5,000 parts per million of phenylurea,diphenylurea, benzoic acid, hydroxy benzoic acid, or mixtures thereof,desirably phenylurea. Such concentrates may also contain silver ions inan amount between about ZOO-5,000 parts per million, preferably betweenabout 5002,S00 parts per million silver ions. The silver ions arepreferably furnished by addition of silver nitrate in an amount betweenabout 3007,000 parts per million, more usually between 750-3,500 partsper million. The etchant solutions are readily prepared from theconcentrates by addition of acid and water and, preferably, otherdesired addi-.

tives such as sulfathiazole. The hydrogen peroxide concentrate may beeasily and safely shipped and has the further advantage of beingstorable for extended periods of time at room temperatures and abovewithout depreciation.

The acid-hydrogen peroxide solutions of the present invention areeminently suited for etching of copper in a highly eflicient andpractical manner. In addition to etching of copper the invention may beapplied generally in other conventional chemical dissolving operationssuch as chemical milling, graining and bright dipping or polishing. Insuch applications the temperature of the acid-peroxide solution may bevaried, if desired, outside of the range prescribed for the etching ofcopper. For example, bright dipping operations may be carried outeffectively at room temperature or slightly above. The additivesprovided by the invention are beneficially effective not only in thepresence of copper but also other metal ions. Thus, the acid solutionscontaining the additives may be employed in the dissolution of othermetals such as iron, nickel.

cadium, zinc, germanium, lead, steel, aluminum and alloys containing amajor portion of such metals. Aluminum metal is more effectivelydissolved when the acid employed is nitric acid or fluoboric acid,particularly fluoboric acid. The solutions are, however, less effectiveon certain other metals such as gold, tin, chromium, stainless steel andtitanium.

The following examples in which parts and percentages are by weightunless otherwise noted demonstrate the practice and advantages of thepresent invention.

of phenylurea and sulfathiazole results in exceptionally fast etch ratesand high capacity markedly superior to Baths A and C containingrespectively phenacetin and phenylurea, and to Bath B in which there wasincorporated a combination of both phenacetin and sulfathiazole. Bath Eshows similarly excellent results when combining phenylurea and silverions. Bath F demonstrates that diphenylurea is substantially as equallyas efiective phenylurea. Bath G shows that 500 parts per million sodiumbenzoate (benzoic acid) is highly effective in providing solutions Thecopper-clad laminates employed in the following 10 of fast etch rate andhigh capacity superiod to Bath C examples were supplied by GeneralElectric Company containing phenylurea alone. Bath H shows furtherimunder trademark Textolite (No. 11571). In the examprovement in thebenzoic acid system upon combination ples the copper laminates were cutinto board specimens with sulfathiazole although the resulting bath fallshort having dimensions of 9 x 9 x A inch. Each specimen had ofachieving of unusually excellent etch rates and capacity 2.7 mil thickcopper (2 ounces per square foot) laminated exhibited by Bath Dcontaining less total amounts of the to a plastic base. Each of thesespecimens was spray etched preferred combination of phenylurea andsulfathiazole. using a Model 600 Spray Etcher manufactured by the Exam yChemcut Division of Centre Circuits Company (U.S.A.). p The reservoir ofthe spray etcher was charged with about TWO dd ti nal elcham Solutionswere P p and 3 gallons of etchant solution and the spray etcher set tofi a d in p y Etching of pp the Same as in the apply about 5 ll perminute to h specimen, Et h preceding examples. Bath I contained asadditives the time was determined with a stopwatch and etch ratecalcombination of 100 parts per million diphenylurea and culated afterweighing each specimen before and after 250 parts per million sodiumsulfathiazole. Bath J contreatment. tained 2,000 parts per million parahydroxy benzoic acid Examples l8 and 200 parts per million silvernitrate. Results are sum- Employing the spray etching apparatus referredto mauled below m Table above eight different etchant solutions wereevaluated by etching a series of the copper-clad specimens with eachTABLE 2 solution. All solutions tested (Baths A-H) contained 6% EtchRate, Minutes by weight hydrogen peroxide and 13.0% by weight sulml BathJ furic acid such that the mol ratio of peroxide to acid wasConcentration-0unces Copper about 1 to 0.75. All etchants were preparedwith deionized j gfig g g 'ggg g fifw -gg; water and contained not morethan 1 part per million total 250 p.p- Benzoic Acid. free chloride andbromide ions. Baths A and B were eva- 35 gfg jg" 200 luated for purposesof comparison. Bath A contained 300 I parts per million phenacetin whileBath B contained a $1, 33? combination of 400 parts per millionphenacetin and 400 mum-e51" 2:0 212 parts per million sulfathiazole.Bath C contained 150 parts per million of phenylurea and Bath Dcontained both 150 40 parts per million phenylurea and sulfathiazole inan amount Of 250 Parts p million- T0 Bath B there was Table 2 shows thatBath I containing the combination added 150 parts per million phenylureaand about 150 of diphenylurea and sodium sulfathiazole exhibits subpartsper million silver nitrate. Bath F contained 150 parts stantially thesame high capacity and fast etch rates as per million of diphenylureawhile Bath G contained 500 the combination of phenylurea withsulfathiazole as emparts per million sodium benzoate. Bath H hadincorployed in Bath D, as reported in Table l. The results poratedtherein a combination of 500 parts per million obtained with Bath Jdemonstrate that the combination sodrumbenzoate and 250 parts permillion sulfathiazole. of hydroxy benzoic acid and silver ions resultsin excep The acid-hydrogen peroxide baths were regulated at a tionallyfast rates and high capacity almost equivalent temperature of about C.during etching. Results sum- 50 to preferred combination of phenylureaand sulfathiazole manzing Examples l-8 are given in Table 1. in Bath Dof Table 1.

TABLE 1 Spray Etch Rate, Minutes gglllxccglsltflggzg: Bath A Bath B BathC Bath D Bath E Bath F Bath G Bath H Diszfillggsoper Aggl) Pei-(glide:Peroxide: Peroxide: Ag?) Peroxice: Acid Peroxide: Acid Peroxide: AcidPeroxide: Acid Peroxide: Etchant militate sutiatiila'iie, M53325suuattli'iie, siit iiitrte, $355513... %..3i3fi' sZtls"i;.-

400 ppm. p.p.m. 150 p.p.m. Benzoate zoate, 250 Phenacetin PhenylureaPhenylurea ppm. Sultathiazole l'ttfitiff" 212 ii; ii iii 21% 21% 11%iii; 4oun0es-- 4.8 2.1 3.5 2.0 2.4 3.5 2.3 2.3 2 o g 2.3 eg 2.3 2.8 5.7as 3.3 10ounees::::-..--:: 14:0 7:6 .fl .331? .11. f1?

'At7ounees of copper.

Table 1 shows the etch rate and capacity of the acidhydrogen peroxidebaths of the invention to be generally very high and at leastapproximately equivalent or supperior to those of the previouslydeveloped additive systerns based on phenacetin. Bath C shows thatsolutions containing only 150 parts per million of phenylurea areclearly superior in both etch rate and capacity to solutions containing300 parts per million of phenacetin, as employed in Bath A. Bath D showsthat the combination Examples 11-16 A series of storage and etchingtests were made to determine ability of the various bath solutions ofExamples l9 to retain maximum effectiveness during periods of non-useafter preparation. In these tests solutions having the composition ofBaths A, B, C, D, F, and G were stored in sealed containers immediatelyafter preparation and portions withdrawn periodically for use in sprayetch- 9 ing of copper according to the procedures and conditionsdescribed in the preceding examples. Results given below in Table 3 aremeasured in terms of loss or reduction of etch rate of the storedsolutions compared with the freshly prepared solution as determined at 6ounces of dissolved copper.

TABLE 3 Percent of Storage Period Maximum Beth and Additives in weeksEflectiveness After Storage Period Bath A-Phenacetin 1 90 BathB-Phenacetin sulfathiazole..- 1 90 Bath CPhenylurea 4 100 Both D-Pheiylurea sultathiazol. 4 100 Bath F-Diphenylurea 4 100 Bath GSodiumbenzoate. 4 100 Examples 17-19 Three bath-holding tests were madeemploying Baths B, D and H. In these tests the freshly preparedsolutions were employed in spray etching as in the preceding examples upto the point where the baths contained 6 ounces per gallon of dissolvedcopper. Etching was then stopped and the solutions held in the etcherover various periods of time after which etching was again commenced andloss or reduction of maximum effectiveness after the holding period wasdetermined. In these tests Baths B and H containing phenacetin andbenzoic acid, respectively, showed a reduction to less than 90% ofmaximum effectiveness after 48 hours while the preferred Bath Dcontaining phcnylurea showed remarkable superiority with a retention ofbetter than 90% of maximum effectiveness on resumption of etching aftera 1 week holding period.

Although certain preferred embodiments of the invention have beendisclosed for purpose of illustration, it will be evident that variouschanges and modifications may be made therein without departing from thescope and spirit of the invention.

We claim:

1. The method for dissolution of a metal selected from the groupconsisting of copper, iron, nickel, cadmium, zinc, steel, aluminum andalloys thereof which comprises contacting the metal at a temperaturewithin the range from about 40 C. up to about 65 C. with an acidifiedaqueous hydrogen peroxide solution having incorporated therein about0.45 to 5.5 grams per liter hydrogen ion and a catalytic amount of anadditive selected from the group consisting of phenylurea, diphenylurea,benzoic acid, hydroxy benzoic acid and salts and mixtures thereof, saidaqueous solution having a total free chloride and bromide ion content ofnot more than 2 p.p.m.

2. The method of claim 1 in which the amount of additive incorporated inthe solution is between about 100 to 1,000 parts per million.

3. The method of claim 1 which the solution also has been incorporatedtherein as catalyst an additive selected from the group consisting ofsulfathiazole phenacetin, silver ions and water-soluble salts andmixtures thereof.

4. The method of claim 1 wherein the additive is phenylurea.

5. The method according to claim 1 in which the solution hasincorporated therein about 2 to 12% by weight hydrogen peroxide.

6. The method according to claim 1 in which the solution hasincorporated therein about 2 to 23% by weight sulfuric acid.

7. The method of claim 1 wherein the additive is present in an amountbetween 20 to 5000 p.p.m.

8. A composition for metal dissolution consisting essentially of anacidified aqueous hydrogen peroxide solution having incorporated thereina catalytic amount of an additive selected from the group consisting ofphenylurea, diphenylurea, benzoic acid, hydroxy benzoic acid, and saltsand mixtures thereof.

9. The composition of claim 8 in which the amount of additiveincorporated therein is between about 100 to 1,000 parts per million.

10. The composition of claim 8 in which there is also incorporated ascatalyst an additive selected from the group consisting of sulfathiazolephenacetin, silver ions and water-soluble salts and mixtures thereof.

11. The composition of claim 8 having incorporated therein between about50 to 1500 parts per million phenylurea or diphenylurea and betweenabout 20 to 1500 parts per million sulfathiazole or salt thereof.

12. The composition of claim 8 having incorporated therein between about50 to 1500 parts per million phenylurea or diphenylurea and betweenabout 10 to 1500 parts per million silver ions.

13. The composition according to claim 8 in which the solution hasincorporated therein about 2 to 12% byweight hydrogen peroxide and about0.45 to 5.5 grams per liter hydrogen ion.

14. The composition according to claim 8 in which the solution hasincorporated therein between about 2 to 23% by weight sulfuric acid.

15. A composition according to claim 8 in which the total chloride andbromide ion content does not exceed 2 p.p.m.

16. The composition of claim 8 wherein the additive is present in anamount between 20 and 5000 p.p.m.

17. The composition of claim 8 in which the additive is phenylurea.

18. A composition suitable for conversion to an acidified-hydrogenperoxide etchant containing 2-12% by weight hydrogen peroxide and acatalytic amount of additive selected from the group consisting ofphenylurea, diphenylurea, benzoic acid, and salts and mixtures thereof,said composition consisting essentially of a concentrated aqueoussolution containing 20-70% hydrogen peroxide and at least one of saidadditives in a total amount between about 200-20,000 parts per million.

19. A composition in accordance with claim 18 in which said concentratedaqueous solution has incorporated therein 30-60% hydrogen peroxide andbetween about 500-5000 parts per million phenylurea.

20. The composition-of claim 18 which additionally has incorporatedtherein a compound selected from the group consisting of sulfathiazole,phenacetin, silver ions, and water-soluble salts and mixtures thereof.

21. The composition of claim 18 which additionally has incorporatedtherein about 200 to 5000 p.p.m. silver ions.

References Cited UNITED STATES PATENTS 3,130,164 4/1964 Best 252- XR3,161,552 12/1964 Bradley et al 156l8 XR 3,269,881 8/1966 Alderuccio etal. 156-3 3,293,093 12/1966 Jones et a] 25279.4 XR 3,341,384 9/1967Alderuccio et al. 25279.4 XR

LEON D. ROSDOL, Primary Examiner.

MAYER WEINBLATT, Assistant Examiner

